Field of the Invention
This disclosure relates to image quality control in an image forming apparatus.
Description of the Related Art
In an image forming apparatus that forms a multi-color image by overlaying toner images of a plurality of colors, a misregistration may possibly occur due to a shift of relative positions of the toner images. One of the factors in a misregistration is a variation of a light irradiation position on a photosensitive member caused by deformation due to the temperature of an optical member. For example, in a configuration in which a polygon mirror is rotated to scan the photosensitive member, the position or posture of an optical member changes due to heat generated by a motor for driving the polygon mirror or heat from peripheral members, and the light irradiation position varies. This variation usually differs among image forming units that form toner images of respective colors, and therefore a misregistration occurs.
For this reason, the image forming apparatus performs calibration for correcting a misregistration. Specifically, a detection pattern for detecting a misregistration amount is formed, the misregistration amount that has occurred is measured by detecting this detection pattern, and an image forming condition is adjusted so as to reduce the misregistration. Printing cannot be performed during calibration, and accordingly the period of calibration is downtime for a user. Moreover, if calibration is frequently performed, each member related to image formation deteriorates, and toner is consumed. Accordingly, the execution frequency of calibration needs to be appropriately set.
Japanese Patent Laid-Open No. 2010-141463 discloses that a calibration execution condition is changed in accordance with a state of use of an image forming apparatus. Japanese Patent Laid-Open No. 2003-207976 discloses that the number of times that calibration is executed is reduced by using both calibration and control for correcting a laser irradiation position based on prediction. Japanese Patent Laid-Open No. 2010-217544 discloses a configuration in which the misregistration amount that has occurred is predicted based on the output of a temperature sensor provided in an image forming apparatus, and calibration is performed in accordance with a prediction result. Specifically, it is disclosed that, if the amount of a temperature change since the previous calibration or the previous prediction is smaller than a threshold value, misregistration correction is performed using a predicted misregistration amount, and calibration is performed if the amount of the temperature change exceeds the threshold value.
An error occurs in a predicted misregistration amount. Accordingly, with a configuration in which the misregistration amount based on a prediction is also used, checking a difference between a prediction value and a measurement value, i.e., a prediction error is important. However, in the configuration of Japanese Patent Laid-Open No. 2010-217544, calibration is not executed unless the amount of a temperature change exceeds the threshold value. That is to say, the misregistration amount that has actually occurred is not measured, and the prediction error cannot be evaluated. In this case, it may be possible that the prediction error has actually become large, and the quality of a formed image has been degraded. Furthermore, the prediction error also varies in accordance with progress of the operation state. For example, FIG. 8A shows an exemplary relationship between the prediction value and the measurement value of a misregistration in the case where an apparatus enters a printing state after a long waiting state, and FIG. 8B shows an exemplary relationship between the prediction value and the measurement value in the case where the printing state and the waiting state are frequently alternated. As shown in FIGS. 8A and 8B, the prediction error after a lapse of the same time differs depending on the operation state of the image forming apparatus. This is because, in general, the prediction error tends to be larger as the prediction value increases and decreases more frequently. However, in Japanese Patent Laid-Open No. 2010-217544, execution of calibration is determined while considering only the amount of a temperature change since the previous calibration or the previous prediction, whereas the operation state is not considered, and the quality of a formed image may possibly be degraded.